Marco's first known target was Len Fletcher, who had been rendered soulless a while back and was in police custody so he could no longer kill anyone. Marco broke into his cell and murdered him there.
Marco's next target was Goldie Schmidtlapp, who was taken to the prencinct after she attempted to kill her own mother. When Marco approached her while she was stuck inside her cell, he found himself bound by a Devil's Trap painted on the ceiling. Sam and Dean then revealed themselves and took him to a motel room, where they began interrogating him.
Marco revealed that Amara was overeating, killing many of his demon pals and also sneaking out to consume human souls. Marco refused to reveal where Amara was being kept, so Dean decided to just kill him. Sam hesitated because he wanted to spare the vessel, but Dean discovered a bullet wound on Marco's chest which meant the vessel was already dead.